The objectives of this research include: (a) Preparation and characterization of low-spin complexes of cobalt with properties related to vitamin B12 and its derivatives, the systematic investigation of the ligand variations including trans-effects on the reactivities of such complexes and the assessment, through appropriate comparisons, of the relevance of such complexes and their reactions as vitamin B12 model systems. (b) Examination of the chemistry of vitamin B12 with particular reference to reactions with alkylating agents leading to the formation of organocobalt derivatives including vitamin B12 coenzymes. (c) Examination of the factors which promote the cleavage of cobalt- carbon bonds in vitamin B12 coenzymes and related organocobalt compounds and mechanistic studies of such cleavage reactions, including thermal homolytic cleavage, oxidative and reductive cleavage and cleavage induced by trans-axial ligand substitutions. (d) Study of the methylation of mercury and other metals by methylcobalamin and related compounds, the mechanistic elucidation of these reactions and their relation to problems of methylmercury pollution of the environment. (e) Through the above studies to attain a better understanding of the mechanistic aspects of the biochemical reactions and roles of vitamin B12 and its derivatives.